The Israelis and the Americans, are not stupid. Especially the Israelis. Nor, are they lying. The attack probably does the bear the marks of Hezbollah. They aren't going to give us the specifics of it, because of the need to safeguard intelligence data, and protect national security.
U.S., allies conduct 22 strikes against ISIS targets in Iraq, Syria (Reuters)
from the article: Pentagon: Terror attack in Bulgaria bears hallmarks of Hezbollah